The display of image data using a raster display is known in the art. Typically, image data is acquired and stored in a digital memory. The image data is subsequently processed and outputted to a raster display where the image appears on the raster display as a sequence of raster line segments. Examples of raster displays include XY recorders, oscilloscope chart recorders, pen recorders, etc.
An existing technique for displaying image data on a raster display includes grouping the image dam in contiguous groups and using the minimum data value and maximum data value in each group to generate a series of line segments on the raster display. For example, FIG. 1 illustrates an analog waveform 20 input signal which is to be displayed on a raster display. The wave/form 20 is sampled and digitized using conventional means to form a set of image data values represented in FIG. 1 by "X"s. The data values are stored in a digital memory and are grouped contiguously with respect to time, for example, to form groups of data values 1-5 as shown. Each group of data values corresponds to the data values which are to be combined to form a raster line segment on the raster display. FIG. 1 illustrates there being three data values in each group, although it will be appreciated that any desired number of data values can be included in each group, the exact number typically being a function of the desired compression ratio.
In order to generate the corresponding line segment on the raster display for each group of data values, the image data is further processed using known minimum-maximum data processing techniques. In particular, the minimum data value and maximum data value for each group of data values is initially determined. For example, FIG. 2 provides a listing of the minimum data value and maximum data value (also referred to herein as the "min-max" pair) for each of groups 1-5. As is noted, the min-max pair for group 1 is 10 and 15, denoted herein as (10,15). Similarly, the min-max pair for group 2 is (12,16), group 3 is (2,12), etc.
The min-max pair for each group is used to produce a raster line segment on the raster display extending from the minimum data value to the maximum data value. More particularly, the min-max pair for each group of data values is used to control the output device on the raster display, e.g., the "pen" on a pen recorder type raster display, the pixels on a digital raster display, etc., in order to produce a series of line segments forming the image represented by the image data. Using this technique, a raster display such as that shown in FIG. 3 is generated. The horizontal axis in FIG. 3 represents the raster line position on the raster display, and each raster line position corresponds to the group of data values used to generate a raster line segment 25 at that particular position. Each raster line segment 25 consists of a line segment extending from the minimum data value to the maximum data value represented by the min-max pair of a corresponding group of data values. As an example, the raster line segment 25 at the raster line position 1 extends from the minimum data value=10 to the maximum data value=15. The raster line segment 25 at the raster line position 2 extends from the minimum data value=12 to the maximum data value=16, and so on. As a result, a more continuous appearing waveform will appear on the raster display as will be appreciated.
There are, however, several drawbacks associated with existing rasterization systems for providing a raster display based on min-max pairs such as described above. For example, FIG. 4 illustrates in part a rasterization system 30 for generating a series of raster line segments on the raster display. Each raster line segment for a given raster line position is represented by a pair of registers 31a and 31b which hold, respectively, the corresponding minimum data value and maximum data value. The registers 31a and 31b each include a corresponding comparison circuit 32a and 2b connected to a common raster scan position counter 35. For each raster line position, the raster scan position counter 35 counts or "scans" through a set of values which correspond to the respective raster scan positions on the vertical axis of the raster display as is shown in FIG. 3.
The comparison circuit 32a compares the value of the minimum data value in the register 31a to the output value of the raster scan position counter 35 and produces a "true" output on line 36a when the value of the raster scan position counter 35 is greater than the minimum data value. Similarly, the comparison circuit 32b compares the value of the maximum data value in the register 31b to the value of the raster scan position counter 35 and produces a "true" output on line 36b when the value of the raster scan position counter 35 is less than or equal to the maximum data value. The outputs of the comparison circuits on lines 36a and 36b are ANDed together by an AND gate 37. As a result, the output of the AND gate 37 is "true" when the raster scan position counter 35 value is within the range of the min-max pair stored in the registers 31a and 31b.
The output of the OR gate 40 is suitable for controlling the pen on the raster display during a line scan, for example, so that the respective line segment or segments, in the case of a multichannel display, for each raster line position will be drawn on a recorder paper. When the output of the OR gate 40 is "true", the pen is placed in the down position so as to contact the paper and to draw a line segment 25 (FIG. 3). When the output of the OR gate 40 is "false", the pen is placed in the up position so as not to contact the paper, thus resulting in a line segment being drawn between the minimum and maximum data values.
To accommodate multiple channels, a pair of registers 31a, 31b and comparison circuits 31a, 31b have been required in the past for each channel of the raster display. The outputs of each AND gate 37 from each respective channel are ORed together by the OR gate 40. Thus, the OR gate 40 produces a "true" output when the value of the raster scan position counter 35 is within the range of the min-max pair for one or more of the channels. On the other hand, the OR gate 40 provides a "false" output whenever the value of the raster scan position counter 35 is not within the range of any of the min-max pairs.
One particular drawback associated with the rasterization system 30 is the requirement of a separate pair of registers and comparison circuits for each channel. As will be appreciated, such a requirement results in increased circuit complexity and cost. For example, the rasterization system 30 requires approximately one hundred logic gates for each channel. As a result, each channel requires substantial circuitry, board space, cost, etc.
Still another drawback associated with existing rasterization systems is that oftentimes it is difficult to distinguish overlapping raster line segments resulting from overlapping images from multiple channels. Referring again to the rasterization system 30 shown in FIG. 4, for example, the output of the OR gate 40 will be the same "true" value regardless of whether the position counter 35 output value falls within the min-max pair of one or multiple channels. Therefore, overlapping line segments are difficult to distinguish.
In view of the aforementioned shortcomings associated with existing rasterization systems, there is a strong need in the art for a rasterization system which is both inexpensive and simple in design. In particular, there is a strong need in the art for a rasterization system which is readily expandable and can accommodate additional channels while only requiring a relatively small increase in resources (e.g., circuitry, board space, cost, etc.). Furthermore, there is a strong need in the art for a rasterization system which can display overlapping line segments from multiple channels so as to be more readily distinguishable as compared to existing rasterization systems.